Living cells and tissues perform and coordinate hundreds to thousands of individual processes, and control the location, orientation, and state of assembly of many structural components in the course of normal life. These processes are usually performed by, and structures are comprised of, specialized groups and classes of molecules. Biologists have used light microscopes to study these processes, both in living cells and in cells that have been preserved at particular points in the cells' lives. Study of these processes or structures involves the detection of molecules or reactions as signals that are often processed and analyzed to help the biologist learn and understand the particular process or structure. Such detection typically relies on a characteristic interaction of light with the molecules responsible for the process or structure that is subject to study. Because components such as molecules are dynamic in living cells and act in concert with, and rely upon, interactions among similar and dissimilar components, it is desirable to study the relationship of a component with one or more other components in a cell.